Netting twine sizes: what #21, 210D/60, tex and millimetres actually mean
A buyer meets the same twine under four names: a US number such as #21, a denier and yarn count such as 210D/60 on a factory quote, a tex or Rtex figure in technical documents, and a diameter in millimetres. They are not four units for one quantity. The US number is a trade size, the denier and tex figures measure mass per length, and the diameter depends on the material and the twist. The most expensive trap is the simplest: nylon #21 is listed as 210/60, not as 210D/21.
What does the # number on sports netting mean?
It is a trade size for the finished twine, and no standards body we found defines it. US sellers describe it by thickness: Practice Sports calls the "#" the "gauge / test" or overall thickness of the netting weave, higher numbers being thicker. For nylon multifilament, net makers also publish the yarn construction behind each number, and on the common sizes they agree with each other.
| US number | Nets & More | Lee Fisher | Fitec | ICNAF 1965, "approximate equivalence" |
|---|---|---|---|---|
| #15 | 210/36 | Not read | 210d/36 | 210 x 36 |
| #18 | 210/48 | 210/48 | 210d/48 | 210 x 51 |
| #21 | 210/60 | 210/60 | 210d/60 | 210 x 66 |
| #36 | 210/96 | 210/96 | 210d/96 | 210 x 99 |
| #42 | 210/108 | 210/108 | 210d/108 | 210 x 108 |
Yarn constructions in 210 denier nylon, read on 2026-10-01: Nets & More, multifilament nylon size and test chart; Lee Fisher, nylon twisted netting No. 21, which labels the number "American Size" and the construction "Int'l Size"; Fitec, twisted knotted multifilament nylon netting; ICNAF Research Document 63, netting twines for trawls (1965). "Not read" means the source was not read for that row, not that the figure is absent.
The older ICNAF table differs by a few yarns on some rows, and it explains why the numbers cannot be decoded by rule: "The low numbers agree with the numbers of folded yarns divided by 3, but this is no longer correct for No. 12 etc." The same document warns that a single trade number is often ambiguous and may vary "from factory to factory".
Is #21 the same as 210D/21?
No, and this is where orders go wrong. A factory quote written in denier and yarn count reads naturally as the US number: "210D/21" looks like #21. It is not. 210D/21 is 21 yarns of 210 denier, 4,410 denier in total, which FAO's table puts at 490 tex. Nylon #21, at 210/60, is 12,600 denier, 1,399 tex in the same table: nearly three times the mass per metre. Fitec lists 210d/21 as #8, and the ICNAF table lists 210 x 21 as No. 7.
Chinese netting factories write twine exactly this way, as denier and ply ("380d/6ply - 380d/240ply", "210D/21") or as a diameter ("2mm"), and rarely as a US number. When your specification is in US numbers and the quote is in denier, convert the quote back to a total denier before comparing, and ask for the diameter of the twine actually offered.
What are denier, tex and Rtex?
- Denier is the mass in grams of 9,000 metres of yarn. 210 denier is the usual nylon netting yarn.
- Tex is the mass in grams of 1,000 metres, the international system defined by ISO 1144. One denier is 0.1111 tex, so 210 denier is 23 tex.
- Rtex, resultant tex, is the mass in grams of 1,000 metres of the finished twine. It is always higher than the yarns added up, because twisting takes up length.
In the tex system a netting twine is written by its yarn and yarn count, with its Rtex: FAO's example is "23/12, R 280 tex", twelve yarns of 23 tex, which the same manual says can also be written "210/12" in denier. ISO 858 is the standard for designating netting yarns this way. When a supplier gives a runnage instead, in metres per kilogram, FAO's formula turns it into Rtex: one million divided by the runnage.
Read on 2026-10-01: ISO 1144, universal system for designating linear density (tex system); FAO Fisherman's workbook, materials and accessories; FAO, management for freshwater fish culture, chapter 8; DeAlteris and Castro, practical twinework for fishermen and gear technologists (URI, NOAA Sea Grant funded).
Why do two suppliers give different millimetres for the same number?
Because diameter is not a conversion. Here is what five sources publish for the same two nylon numbers:
| Source | Nylon #21 | Nylon #36 |
|---|---|---|
| Nets & More | 1.65 mm | 2.16 mm |
| Fitec, netting chart | 1.7 mm | 2.16 mm |
| Lee Fisher | 1.79 mm | 2.29 mm |
| Practice Sports | 2.0 mm | 2.5 mm |
| Gourock | "estd. 2.0mm" | "estd. 2.5mm" |
Read on 2026-10-01 on the sources linked in this guide; Practice Sports product pages and Gourock, #21 knotted nylon netting. Gourock's own wording, "estd.", marks its millimetres as estimated.
The spread is not careless publishing. The 1965 ICNAF document states the cause: twines of the same Rtex made of fibres of different density can differ in diameter, and so can twines of the same fibre with a different hardness of twist or braid. FAO's tables show it in numbers: nylon twine of Rtex 4,000 is about 2.4 mm twisted and 3.10 mm braided, and at roughly the same Rtex, about 1,400, polyester comes out at 1.20 mm, nylon at 1.40 mm, polyethylene at 1.64 mm and polypropylene at 1.70 mm, the densest fibre being the thinnest. ICNAF's own summary: such diameters are "very often only nominal".
Does #36 mean the same thing in nylon, polyethylene and polyester?
No. The constructions above are for nylon. For polyethylene the same numbers are built from heavier yarns: Lee Fisher lists its polyethylene twisted twine as "21 (380D/33)" and "36 (380D/54)", and Jones Sports describes its HDPE batting cage nets as "#24 Twine Size HDPE (42PLY)". We found no number table for polyester at all. Since the netting our manufacturing partner documents is polyester, specified in millimetres, any US number we give for it is a cross reference by diameter against nylon tables, and we say so.
What does a break strength figure refer to?
It depends on what was broken, and the standards make the distinction that sellers usually leave out. ISO 1805 tests the "breaking force and knot breaking force of netting yarns", dry or wet, and considers the wet knotted test "particularly appropriate in indicating the behaviour of the yarn in use". ISO 1806 tests the mesh breaking force of netting. FAO's tables give two figures per twine, dry without knots and wet with a knot, and for nylon the second is roughly half the first.
Sellers who qualify their figures do it in different ways: Nets & More publishes a twine column and a knot column, 190 lb and 125 lb for nylon #21; Lee Fisher's netting figures are a "Knotted Break Strength"; Batting Cages Inc gives its HDPE figures as "breaking strength per strand"; Gourock gives a "single twine breaking strength" minimum. None of the sellers we read cites ISO 1805 or ISO 1806. Two break figures can therefore only be compared once each states the material, what was broken, and whether it was wet or dry. Our twine gauge chart keeps each published figure next to its source for that reason.
What should the specification say?
- Material and construction first. "Polyester, twisted" or "nylon, braided": without them a diameter or a number means little.
- Diameter in millimetres, as measured on the twine offered. Not converted from a table.
- The construction in denier or tex, when the factory works that way. Total yarns as well as yarn size: "210D/60", never "210D" alone.
- The US number only as a cross reference. "2.0 mm, quoted elsewhere as #21", with the material named.
- Break strength with its test. Twine, knot or mesh; wet or dry; and the method, if the supplier has one.
The twine is one field of the free quote-comparison matrix and RFQ checklist; the mesh it is knotted into is covered in how netting mesh size is measured. For batting cages, where the number matters most to buyers, see batting cage twine gauge by level. The combinations our partner documents are in the Spec Finder and under netting by the metre.
Frequently asked questions
Is #21 twine the same as 210D/21?
No. For nylon multifilament, the three net makers whose charts we read put #21 at 210/60, sixty yarns of 210 denier. A twine written 210D/21 has 21 yarns, about a third of the mass per metre, and is listed at about #8 by one US maker and as No. 7 in a 1965 fisheries commission table. Ordering "210D/21" to replace a #21 net would buy a much finer twine.
How do I convert 210D/60 into tex?
Multiply the total denier by 0.111, the factor ISO 1144 uses. 60 yarns of 210 denier make 12,600 denier, and FAO's table gives 1,399 tex for that construction. That is the nominal figure for the yarns before twisting; the finished twine weighs more per kilometre, which is what Rtex measures.
Why is Rtex always higher than the sum of the yarns?
Because twisting shortens the yarns: a kilometre of finished twine contains more than a kilometre of each yarn. A US fisheries manual puts it plainly, the R-tex of a twisted product "is always a larger value; the harder the twist, the larger the value". FAO suggests adding about 10 percent to the calculated figure as a rough estimate, and using a measured Rtex for braided twines.
Can I order twine by diameter alone?
Only if you also state the material and the construction. Twines of the same weight per metre come out at different diameters in nylon, polyester, polyethylene and polypropylene, and a braided twine is thicker than a twisted one of the same Rtex. A diameter is a measurement of one product, not a conversion that holds across suppliers.
Holding a quote in denier and a specification in US numbers?
Send both as they stand. The reply within 2 business days states the total construction of each, which documented combination is closest by diameter, and what is still missing before the two can be compared.
We reply within 2 business days with questions or a documented quote.